The present invention relates to an installation and a method for venting the waste gases of air distillation or liquefaction units and can especially be used in those of the said venting installations which comprise a stack, for example made of concrete, and a water-nitrogen tower.
In this text the word xe2x80x9coxygenxe2x80x9d must be interpreted as comprising fluids containing at least 75 mol % oxygen and preferably at least 95 mol % oxygen, the word xe2x80x9cnitrogenxe2x80x9d must be interpreted as comprising fluids containing at least 90 mol % nitrogen, preferably at least 95 mol % nitrogen, and the word xe2x80x9cargonxe2x80x9d must be interpreted as comprising fluids containing at least 60 mol % argon.
During implementation of air distillation and liquefaction processes, in units handling large amounts of air (substantially greater than or equal to 1000 tonnes per day) the water which is intended to cool the incoming air upstream of the purification units is itself cooled in a water-nitrogen tower, at the upper part of which the water is injected, by heat exchange with dry nitrogen. The resulting wet nitrogen is then continuously or at least almost continuously vented at the top of the tower. Moreover, for the venting of the contaminated or uncontaminated products coming from the air distillation or liquefaction, including oxygen, these are collected in a concrete stack and discharged via the top of the stack.
To make the venting installation more compact and facilitate its construction, it is possible to place the stack alongside the water-nitrogen tower.
Such a venting installation is drawn schematically in FIG. 1, which shows a cross section of this plant in a vertical plane.
In this FIG. 1, an installation for venting the waste gases from an air distillation or liquefaction unit comprises a water-nitrogen tower 1 and a stack 2 joined together by a common partition 3.
The water-nitrogen tower includes, near its base, a dry nitrogen feed pipe 11 and, in its upper part, a pipe 12 for feeding hot water to be cooled, which water is itself intended to then cool the air as mentioned above; thus, in the tower 1 the nitrogen follows a generally upwardly vertical path up to an upper chamber 13 having, at the top of it, an outlet 14 for discharging wet nitrogen into the atmosphere, whereas the cold water 16, pumped into the lower part of the tower, is sent to the units where it is brought into contact with the air to be cooled. The stack 2 includes, near its base, pipes for feeding waste gases to be discharged into the atmosphere, for example an air pipe 21, a nitrogen pipe 22 and an oxygen pipe 23; the stack 2 includes, near the top of it, a waste gas discharge outlet 24.
It may be noted that the wet nitrogen vented by the water-nitrogen tower 1 is discharged continuously. On the other hand, at various steps in the process it is possible that only a single waste gas is vented by the stack 2. The design of the stack must of course be chosen so that it is able to vent the three gases simultaneously, a situation which may occur; thus, the stack is designed to vent the gases with a total maximum flow rate corresponding to an exit velocity typically of about 20 m/s. However, this design is not devoid of drawbacks; if only a single gas is vented, because of the low flow rate, the venting velocity is very much less than the chosen value, in this case 20 m/s; this may have very unfortunate consequences if the single vented gas is oxygen: since oxygen is heavier than air, shortly after it has exited the stack there is a tendency for it to come back down towards the ground, and in certain weather conditions (for example when there is almost no wind), its level of concentration at ground level may locally become greater than 25%, which is excessive since it is dangerous especially on account of the oxidant properties of this gas.
The invention explained below remedies this drawback and for this purpose consists of an installation for venting the waste gases of an air distillation or liquefaction process, characterized in that it comprises a waste gas discharge stack emerging in the atmosphere and suitable for discharging in particular oxygen at least intermittently and means for reducing the level of oxygen concentration in the gases discharged by the stack, these possibly consisting of means for mixing an inert gas, miscible with oxygen and of lower density than oxygen under the same temperature and pressure conditions, preferably of lower density than air under the same temperature and pressure conditions, with the gases discharged.
Preferably, the installation comprises a chamber for almost permanently discharging into the atmosphere the inert gas miscible with oxygen and of lower density than air under the same temperature and pressure conditions, and means for connecting the respective internal spaces of the discharge chamber and of the stack in order to transfer at least some of the said inert gas into the stack so that the inert gas is mixed with at least the oxygen in the stack, and thus the level of oxygen concentration of the gases discharged by the stack is reduced.
By virtue of this arrangement, it is possible to appreciably reduce the level of oxygen concentration in the gas emitted at the outlet of the stack, even if only oxygen is introduced near the base of the stack.
The installation may furthermore have one or more of the following characteristics:
the discharge chamber is a chamber forming part of a water-nitrogen tower;
the inert gas is nitrogen or oxygen or air or a mixture of these gases;
the discharge chamber forms part of a water-nitrogen tower placed alongside the stack, and the internal spaces in the chamber and in the stack are separated by a partition having, as means for connecting the internal spaces, an outlet for discharging, into the stack, the wet nitrogen contained in the chamber;
the stack is equipped internally with a set of nozzles through which some or all of the gas introduced into the base of the stack flows;
the connecting means comprise a discharge outlet provided in a partition separating the internal spaces in the chamber and in the stack, and the stack is equipped internally with a set of nozzles arranged in such a way that the top of it is at a level below the top of the discharge outlet;
the discharge chamber belongs to a water-nitrogen tower having, near its base, a dry nitrogen feed pipe and, in its upper part, a pipe for feeding the hot water to be cooled, above the level of which a wet nitrogen discharge outlet, opening into the stack, is provided; and
the stack includes, near its base, an air feed pipe and/or a nitrogen feed pipe and/or an oxygen feed pipe and/or a pipe for feeding another gas coming from the distillation.
The invention also consists of a method for venting oxygen-containing waste gases via a discharge outlet of a stack of an air distillation or liquefaction unit, characterized in that wet or dry nitrogen is mixed with at least the oxygen and the oxygen/nitrogen mixture obtained is discharged with a velocity at least equal to approximately 7 m/s at the discharge outlet.
As a variant:
the oxygen/nitrogen mixture obtained is discharged with a velocity at least equal to 10 m/s at the discharge outlet;
the oxygen/nitrogen mixture obtained is discharged with a velocity at least equal to approximately 12 m/s at the discharge outlet; or
the oxygen/nitrogen mixture obtained is discharged with a velocity at least equal to approximately 20 m/s at the discharge outlet.